Network synchronization is essential for TD-LTE systems, and it is beneficial also for FDD-LTE systems if advanced features are utilized, such as eICIC, CoMP, eMBMS, or advanced IC receiver application. Practically, the network synchronization can be achieved with satellite solution (e.g. GPS or GNSS), network backhaul solution (e.g. NTP, SyncE, or PTP), or over-the-air/radio-interface based solution (e.g. Network Listening Mode). For each of the synchronization solutions, there are pros and cons in terms of frequency/phase synchronization accuracy, hardware cost and/or applicability. Radio interface based (RIB) network synchronization has been seen as an option when other synchronization techniques are costly or deemed unavailable due to, for example, lack of satellite coverage (e.g., no GPS/GNSS coverage) or poor quality network backhauling (e.g. with poor quality network backhaul link involving routers and/or switches).
FIG. 1 shows an example diagram of RIB synchronization chain.
When for example a small cell like an eNB/HeNB (e.g., an “unsynchronized cell” 102 as shown in FIG. 1) is powered-up, it will search for radio interface signals from other cells to find a synchronous cell with lowest stratum-level with sufficient SNR as its source cell for synchronization. By knowing the stratum-level of a source cell, the small cell will decide its own stratum-level by simply adding 1 to the stratum level of the selected source above, e.g. if the stratum-level of source cell is n, then stratum-level of the listening small cell is simply n+1. As yet another cell may acquire its synchronization later on from the aforementioned small cell, cells are seen to form synchronization chains.
A typical network synchronization requirement for radio-interface based (RIB) application is 3 μs (or ±1.5 μs) between any two overlapping cells. If the two overlapping cells belong to different synchronization chains, the synchronization error between the root cell (i.e., the cell 101 at stratum 0 in FIG. 1) of a synchronization chain and a small cell (e.g. the “unsynchronized cell” 102 in FIG. 1) at the end of the synchronization chain must be less than half of the requirement value (3 μs/2=1.5 us) in order to guarantee that the network synchronization requirement for RIB application is fulfilled. By considering dense small cell deployment scenario the supported maximum hop level may be further extended to for example even 6 hops or 7 stratum levels. Practically, the number of hops that can be supported for each synchronization chain depends on the synchronization accuracy accumulated over each hop. Based on the legacy RIB scheme, a signal-noise-ratio (SNR) threshold is generally used for each hop in order to guarantee that the synchronization requirement could be met with the maximum number of supported hops.
However, in reality, the choosing of the appropriate SNR threshold will be a difficult task. If a tight SNR threshold is utilized, a maximum number of hops could be supported while ensuring sufficient synchronization accuracy over the whole synchronization chain, but the portion of cells synchronized with RIB will be quite limited due to the tight SNR threshold applied. With a considerable probability, a cell acquiring a RIB synchronization simply cannot find a synchronization source cell with signal quality exceeding the SNR threshold. On the other hand, if a more less tight SNR threshold is used, there will be a higher number of successfully synchronized RIB cells with a stratum level equal or below a certain value. But the uncertainty on the actual synchronization accuracy increases especially for cells with a high stratum level.